Three piston rings: a top ring, a second ring, and an oil ring those used in an internal combustion engine are disposed in such a manner that each of them engages with a piston ring groove provided on a surface of a piston and serve a gas sealing function of preventing combustion gas from leaking out from a combustion chamber, a heat conduction function of transmitting heat of the piston to a cylinder wall which is cooled down and thereby cooling the piston down, and a function of scooping out excess oil by applying an appropriate amount of engine oil serving as lubrication oil to the cylinder wall, respectively.
Those three piston rings, during operation of the internal combustion engine when the piston is reciprocated by explosion of fuel in the combustion chamber, repeat collision against an inner wall surface of the piston ring groove of the piston ring. Also, the piston rings are slidable inside the piston ring groove in a circumferential direction thereof and slide inside the piston ring. However, the piston ring groove has projections having a height of approximately 1 μm on a surface thereof generated by machine turning in forming the groove. Therefore, the projections wear due to the sliding collision of the piston ring, causing exposure of an aluminum surface on the surface of the piston ring groove.
When the exposed aluminum surface comes into contact with a piston ring side face by the collision and the piston ring further repeats the sliding, aluminum cohesion, a phenomenon that aluminum alloy is adhered to the piston ring side face, occurs. This phenomenon is especially noticeable at the top ring located proximate to the combustion chamber and exposed to high temperature.
When the aluminum cohesion further progresses, wear of the piston groove rapidly progresses and the piston ring becomes fixed to the ring groove. As a result, the gas sealing function of the piston ring is degraded and causes a phenomenon what is called blow-by that high pressure combustion gas flows out to a crank chamber from the combustion chamber, leading a reduction in engine power.
As such, there have been suggested a variety of techniques for preventing the aluminum cohesion of the piston ring. PTL 1, for example, describes a technique for preventing the aluminum cohesion by providing a resin-based film containing carbon black particles on the side face of the piston ring that slides and collides against the piston ring groove, thereby improving conformability.
Also, PTL 2 describes a technique for effectively preventing the piston ring from the aluminum cohesion by providing a heat-resistant resin containing, relative to an entire surface film, 10 to 80 mass % of one or more powder selected from a group including nickel-based powder, lead-based powder, zinc-based powder, tin-based powder, and silicon-based powder to at least one of an upper side face and a lower side face of the piston ring.
However, the films described in PTLs 1 and 2 have a problem that, when temperature inside an engine rises, aluminum cohesion resistance is degraded. As such, PTL 3 describes a technique that provides a polyimide film containing hard particles and having a solid lubricant function to at least one of the upper side face and the lower side face of the piston ring, thereby maintaining high aluminum cohesion resistance for a long time under high temperature conditions over 230° C.
Further, PTL 4 describes a technique that provides, instead of the resin-based film, a first diamond like carbon (DLC) film containing at least silicon and a second DLC film formed under the first DLC film and containing at least W, or W and Ni, to the upper side face and the lower side face of the piston ring, thereby providing a piston ring having excellent aluminum cohesion resistance, scuff resistance and abrasion resistance.